BTW: Paul calls this MYSTERY of the gospel "the gospel of the grace of God" (Acts 20:24); "the preaching of the cross" (1 Cor. 1:18), and "my gospel"(Rom. 2:16, 16:25) and "the gospel of the uncircumcision"(Gal. 2:7) and "that gospel I preach amongst the Gentiles"(Gal. 2:2).
Acts 20:24 But none of these things move me, neither count I my life dear unto myself, so that I might finish my course with joy, and the ministry, which I have received of the Lord Jesus, to testify the gospel of the grace of God.
There you have it. The extent of the grace of God was not revealed until it was given to Paul. Throughout the Old Testament and through the death of Christ action was needed. There were sacrifices, feast days, circumcision and many other rules and conditions. Not until after Christ ascended to heaven and the Holy Spirit had been given was there any understanding of the free gift of salvation through no merit on mans part. That was the revelation that was given to Paul. The other apostles didn't get that revelation untill Paul told them. That's why Peter said it was "hard to understand". The Jews didnt understand the change that occurred at Christs death and resurrection and many Christians today dont understand the change that occurred when the Holy Spirit was given. That was the revelation that was given to Paul.